1. Field of the Invention
This invention relates to electronic musical instruments and in particular to means for eliminating undesirable keying transients in such instruments.
2. Description of the Prior Art
In electronic musical instruments, such as electronic organs, a desired output sound is obtained by selective depressions of keys which effectively start and stop delivery to the amplifier of signals generated by a plurality of tone generators. The sudden starting and stopping of the signal delivery to the amplifier introduces into the transmitted signal spurious side band frequencies which produce audible clicks and thumps in the output sound. These keying transients are annoying and substantial effort has been expended in an attempt to eliminate or reduce them to an acceptable level.
Another cause of key clicks and thumps in such instruments is the DC offset generated by conventional electronic keyers. Conventional methods to eliminate these transients include relatively costly balanced modulators, field effect transistors in series shunt arrangements, and light dependent resistors.
The problem of such keying transients generally was considered in U.S. Pat. No. 2,228,119 of Theodore P. Kinn relative to the production thereof in radio telegraphy. As pointed out in that patent, it was well recognized that the undesirable key clicks were caused by a high order of side band frequencies produced by the sudden starting and stopping of the oscillations. The patentee indicates that in order to eliminate the cause of such key clicks, it is necessary to provide some means whereby the oscillations are controlled to start and stop gradually instead of abruptly. To do this, the patentee teaches the use of reactive elements for modifying the shape of the transmitted wave sound and forming a critically damped oscillatory circuit.
An early patent utilizing this method of reducing the keying transients in electronic organs was that of Leslie A. Bourn U.S. Pat. No. 2,522,923 wherein condensers were provided in combination with resistors for slowing down the rate of increase of the keyed signal.
The soft keying concept is disclosed in detail in U.S. Pat. No. 2,089,781 of Werner Buschbeck. As pointed out therein, it was conventional to use electrical or mechanical retarding means to operate a modulation stage with a keying lag. In the structure of the patent, a transmitter stage controlled by a key is gradually keyed in and out through the intermediary of suitable retarding means to ensure the desirable soft keying.
In the Stanley Cutler U.S. Pat. No. 3,514,723 owned by the assignee hereof, a tone control circuit is provided utilizing a potentiometer for selectively attenuating or boosting either the high or low frequencies with respect to a given reference frequency. The circuit differentially adjusts the network's transmission characteristics to function as either a low pass filter, a high pass filter, or a frequency-flat transmission channel.
The use of delaying circuitry to provide soft keying by reducing the number of side band frequencies is not completely satisfactory in that this technique introduces the disadvantage of preventing an instantaneous response to the player's operation of the keys. Such soft keying produces a time delay effect which is completely different from the normal instantaneous effect, e.g., as obtained in the playing of a conventional piano wherein the sound appears substantially simultaneously within the striking of the key. Even a small delay in the buildup of the sound is sensed by the player of the organ so as to make the organ seem somewhat unresponsive. Thus, while reducing the click problem, the soft keying technique introduces a different, unnatural response problem.